Conventional communication antennas are mechanically bulky with complex steering systems that make mobile operations difficult. One solution to this problem is a Phased Array Antenna (PAA) which has been recognized as the preferred choice for high gains, with electronic beam-steering that is capable of maintaining a continuous link between mobile platforms.
The microwave modules of the PAA are the most critical component of the antenna, and it is the module design that determines the PAA performance and cost.
PAA transceiver modules employ multi-layer ceramic substrates for mounting MMICs and beam steering control Integrated Circuits (ICs). The ceramic substrate provides the necessary microwave signal paths between module and waveguide, as well as providing electrical circuit connections for power and control signals. The very same ceramic substrate also serves as the thermal path between the MMIC amplifiers and the module base plate where power amplifiers are employed.
However, MMIC (in this case, mostly gallium arsenide (GaAs) Integrated Circuits) performance degrades rapidly in the presence of moisture. Degradation can be observed through measurement of signal attenuation, RF loss, and parameter shift. In particular, condensation is a major concern if it occurs on the surface of the MMICs; it can lower device surface temperature and result in dendrite formation. For example, when gold is exposed to humidity under electrical field stress, dendrites can grow large enough to bridge insulation within just a few days.
Thus, hermeticity of the PAA transceiver modules is crucial to avoid degradation and losses. However, the packaging of the PAA transceiver modules to meet hermeticity requirements has resulted in cost drivers which have limited the application of PAA technology.
The current packaging approach involves placement of the semiconductor die in a hermetically sealed, customized, multi-layered, high cost, high-temperature co-fired ceramic package with thick film metallization. Aside from the inherent high material cost, this packaging configuration also requires manufacturing processes that do not lend themselves to low-cost, high volume processing. Further, due to the large number of hermetic PAA modules (each of the modules which are individually hermetically sealed and brazed to a metal can structure), there is a high cost involved.
Further, wide-bandwidth microwave electronics require the use of novel interconnect and packaging techniques; and thus, lightweight, wide-bandwidth PAA systems are needed to enable the space-based reconnaissance concepts of the future. For example, applications of wide-bandwidth PAA systems would include advanced airborne surveillance platforms that can be installed on various vehicles. These systems employ Electromagnetic Support Measures (ESM), broadband communications and advanced radar systems.
For narrow-band applications, MMICs and their control electronics often use bond wires. Since bond wires introduce extra inductance, they are actually serving the purpose of frequency tuning in narrow-band applications. However, obtaining precise wire length for a particularly turned frequency requires tight tolerance control during production.
Further, bond wires are not suitable for, for example, 2–18 GHz wideband applications. For wide-band applications, extra inductance of the bond wires degrades the RF performance.
Accordingly, due to the customized nature of the antenna module design and its tight tolerance requirement, the cost of a complete RF module can be very high. For example, for low volumes, 85% of the total package cost is associated with the high-temperature, co-fired ceramic package and its attachment to the metal header.
As an alternative and replacement to bond wires, die bumps are used in a die attachment known as the flip-chip method. However, the flip-chip method does not work well with MMICs that have been designed using conventional RF design and layout rules. Microwave structures such as micro-strips and strip-lines have unique RF characteristics depending on their spacing and the media above and ground plane below them. Currently, there are only a few design and layout tools to support flip-chip mounted microwave devices under development.
Accordingly, a commercial near-hermetic concept for the development of materials, packaging, and manufacturing technology used in wide-band RF systems (example, 2–18 GHz), which meets system requirements and can reduce component cost by more than 30%, is required.